U.S. patent application number 15/788088 was filed with the patent office on 2019-04-25 for method, tire-mounted tpms component, and machine readable storage or computer program for determining a duration of at least one contact patch event of a rolling tire.
The applicant listed for this patent is Infineon Technologies AG. Invention is credited to Benjamin Kollmitzer, Christoph Steiner.
Application Number | 20190118592 15/788088 |
Document ID | / |
Family ID | 65996711 |
Filed Date | 2019-04-25 |
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United States Patent
Application |
20190118592 |
Kind Code |
A1 |
Kollmitzer; Benjamin ; et
al. |
April 25, 2019 |
Method, Tire-Mounted TPMS Component, and Machine Readable Storage
or Computer Program for Determining a Duration of at Least one
Contact Patch Event of a Rolling Tire
Abstract
Examples provide a method, a component, a tire-mounted TPMS
module, a TPMS system and a machine readable storage or computer
program for determining a duration of at least one contact patch
event of a rolling tire. A method for determining a duration of at
least one contact patch event of a rolling tire, comprises
obtaining a sequence of acceleration measurement samples of the
rolling tire from a tire-mounted acceleration sensor; and
determining the duration of the contact patch event based on
acceleration measurement samples of the sequence between a first
time instance when the acceleration measurement samples cross a
first threshold and a second time instance when the acceleration
measurement samples cross a second threshold.
Inventors: |
Kollmitzer; Benjamin; (Graz,
AT) ; Steiner; Christoph; (St. Margarethen,
AT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Infineon Technologies AG |
Neubiberg |
|
DE |
|
|
Family ID: |
65996711 |
Appl. No.: |
15/788088 |
Filed: |
October 19, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B60C 23/0489 20130101;
B60C 23/064 20130101; B60C 23/061 20130101; B60C 2019/004 20130101;
B60Y 2400/304 20130101 |
International
Class: |
B60C 23/06 20060101
B60C023/06 |
Claims
1. A method for determining a duration of a contact patch event of
a rolling tire, comprising: obtaining a sequence of acceleration
measurement samples of the rolling tire from a tire-mounted
acceleration sensor; and determining the duration of the contact
patch event based on acceleration measurement samples of the
sequence between a first time instance when the acceleration
measurement samples cross a first threshold and a second time
instance when the acceleration measurement samples cross a second
threshold.
2. The method of claim 1, wherein a slope of the sequence of
acceleration measurement samples crossing the first threshold is of
different sign than the slope of the acceleration measurement
samples crossing the second threshold.
3. The method of claim 1, wherein at least one of the first and the
second threshold corresponds to an average value of the
acceleration measurement samples obtained during one or more
revolutions of the rolling tire.
4. The method of claim 1, wherein the first and the second
threshold are different from each other.
5. The method of claim 1, wherein the first time instance is
smaller than the second time instance and wherein the first
threshold has a smaller absolute value than the second
threshold.
6. The method of claim 1, wherein determining the duration
comprises determining the first time instance when the acceleration
measurement samples cross the first threshold, determining the
second time instance when the acceleration measurement samples
cross the second threshold after the first time instance, and
determining the duration from a difference between the first and
the second time instance.
7. The method of claim 1, wherein the duration is determined based
on the number of samples between the first and the second time
instance and a known sampling rate.
8. The method of claim 1, wherein the first threshold equals the
second threshold, wherein determining the duration comprises:
determining a difference between the first or the second threshold
and each sample of the sequence of acceleration measurement
samples; accumulating the difference into an accumulated sum;
setting the accumulated sum to zero whenever the accumulated sum is
negative; stopping accumulating the accumulated sum when the
sequence of acceleration measurement samples reaches the second
time instance; and dividing the accumulated sum by the difference
between the second threshold and an acceleration value
corresponding to zero acceleration.
9. The method of claim 8, wherein the first time instance is
updated to the time corresponding to the sample that caused the
accumulated sum to be set to zero; and wherein the duration is
determined from the difference between the first and second time
instances after the accumulation has stopped.
10. The method of claim 1, wherein determining the duration
comprises determining a weighted integral of the acceleration
measurement samples between the first and the second time
instance.
11. The method of claim 1, wherein the first threshold equals the
second threshold, and wherein determining the duration of the
contact patch event comprises: determining the first and the second
time instance by extremizing an integral of the sequence of
acceleration measurement samples; and determining the duration of
the contact patch event by dividing the value of the integral by a
difference between the first or the second threshold and an
acceleration value corresponding to zero acceleration.
12. The method of claim 1, wherein obtaining a sequence of
acceleration measurement samples comprises obtaining, processing,
and discarding a first set of measurement samples before a
subsequent set is obtained.
13. The method of claim 8, wherein determining the duration
comprises obtaining, processing, and discarding a first set of
measurement samples before a subsequent set is obtained.
14. The method of claim 1, further comprising: estimating a time
window of the subsequent contact patch event based on the sequence
of acceleration measurement samples, wherein the estimated time
window comprises at least two time instances corresponding to the
first and second time instance of a subsequent contact patch event;
and increasing a sample rate of the sequence of acceleration
measurement samples during the estimated time window with respect
to a reduced sample rate outside the estimated time window.
15. The method of claim 14, wherein estimating the time window of
the subsequent contact patch event of the rolling tire comprises:
determining a rotational rate of the tire; identifying a sample
within the sequence of acceleration measurement samples of the
rolling tire, indicative of a minimum radial acceleration; and
estimating a time window of the subsequent contact patch event of
the rolling tire based on the identified sample and the rotational
rate of the tire.
16. The method of claim 15, further comprising validating the
estimated time window, wherein the method is aborted if the time
window exceeds a predetermined threshold.
17. The method of claim 1, further comprising validating the
sequence of samples, wherein the method is aborted if the samples
exceed a predetermined threshold.
18. The method of claim 1, further comprising validating the
determination of the duration of the contact patch event, wherein
the method is aborted if the duration exceeds a predetermined
threshold.
19. The method of claim 18, wherein validating the determination of
the duration of the contact patch event further comprises:
comparing at least two determinations of the duration of at least
one contact patch event wherein each of the at least two
determinations is obtained with a different method; and aborting
the method if the at least two determinations differ by more than a
predetermined threshold.
20. The method of claim 19, wherein the at least two determinations
of the duration comprise: a first determination obtained by:
determining the first time instance when the acceleration
measurement samples cross the first threshold, determining the
second time instance when the acceleration measurement samples
cross the second threshold after the first time instance, and
determining the duration from a difference between the first and
the second time instance; and a second determination obtained by:
determining a difference between the first or the second threshold
and each sample of the sequence of acceleration measurement
samples; accumulating the difference into an accumulated sum;
setting the accumulated sum to zero whenever the accumulated sum is
negative; stopping the accumulation of the accumulated sum when the
sequence of acceleration measurement samples reaches the second
time instance wherein the determination of the duration of the
contact patch event comprises: dividing the accumulated sum by the
difference between the first threshold and an acceleration value
corresponding to zero acceleration.
21. A tire-mounted TPMS component, comprising: a tire-mounted
acceleration sensor, the acceleration sensor being configured to
generate a sequence of acceleration measurement samples of the
rolling tire; and an electronic control unit configured to
determine the duration of the contact patch event based on
acceleration measurement samples of the sequence between a first
time instance when the acceleration measurement samples cross a
first threshold and a second time instance when the acceleration
measurement samples cross a second threshold.
22. The tire-mounted TPMS component of claim 21, further
comprising: wherein the electronic control unit is further
configured to: estimate a time window of the subsequent contact
patch event based on the sequence of acceleration measurement
samples, wherein the estimated time window comprises at least two
time instances corresponding to the first and second time instance
of a subsequent contact patch event; and wherein the sensor is
further configured to: increase a sample rate of the sequence of
acceleration measurement samples during the estimated time window
with respect to a reduced sample rate outside the estimated time
window.
23. A machine readable non-transitory storage including machine
readable instructions to determine a duration of a contact patch
event of a rolling tire, that when executed: obtains a sequence of
acceleration measurement samples of the rolling tire; and
determines the duration of the contact patch event based on
acceleration measurement samples of the sequence between a first
time instance when the acceleration measurement samples cross a
first threshold and a second time instance when the acceleration
measurement samples cross a second threshold.
Description
FIELD
[0001] Examples relate to tire pressure monitoring systems (TPMS)
and to angular position sensing (APS), in particular but not
exclusively, to a method, a tire-mounted TPMS component, and a
machine readable storage or computer program for determining a
duration of at least one contact patch event of a rolling tire.
BACKGROUND
[0002] Tire pressure monitoring systems are traditionally used in
automotive applications to monitor the inflation pressure of
vehicle tires and to warn the driver in case of abnormal
inflation.
[0003] For direct TPMS, modules--comprising at least of a sensor,
control logic, a radio frequency (RF) transmitter and a source for
electrical energy--are mounted in a tire. Each module measures the
inflation pressure and transmits this value together with module
identification (ID) via RF to the electronic control unit (ECU) in
the vehicle.
[0004] Standard TPMS modules are valve-based, i.e. mounted on the
valve and thus fixed to the rim. In contrast to valve-based TPMS
modules, tire-mounted modules are mounted in the tire cavity on the
inner liner of the tire.
[0005] With valve-based TPMS modules, one can infer the angular
position from the direction of the earth's gravity, which is
measured with accelerometers. The accelerations acting on such
modules comprise mainly the centrifugal acceleration due to the
spinning wheel, mechanical vibrations, and the earth's gravity.
[0006] Due to the more flexible mounting position of tire-mounted
TPMS modules, the relevant sources for accelerations are different.
As the tire spins during vehicle movement, such tire-mounted TPMS
modules follow roughly a trajectory determined by the tire's
circumference. In the vehicle-frame, i.e. a coordinate system which
is fixed to the vehicle, the tires' circumferences and thus the
trajectories resemble flattened circles, where the flat is
determined by the contact patch (footprint) between the tire and
the ground. Tire-mounted TPMS modules are thus subject to
fundamentally different acceleration waveforms than valve-based
TPMS modules.
[0007] Knowledge of the contact patch length is of interest because
it enables load detection: The tire can only transfer loads via the
contact patch to the road. Because the tire is flexible, the area
of the contact patch varies with the applied loads, the inflation
pressure etc. Vice versa, knowledge of the contact patch, inflation
pressure, and the mechanical properties of the tire allows the
vehicle to estimate the acting tire load. This information could
potentially increase safety, energy efficiency and comfort; e.g. by
detecting overload, adjusting the suspension and suggesting
appropriate inflation pressures for optimal traction and CO.sub.2
efficiency.
SUMMARY
[0008] Examples relate to tire pressure monitoring systems (TPMS)
and to angular position sensing (APS), in particular, but not
exclusively, to a method, a tire-mounted TPMS component, and a
machine readable storage or computer program for determining a
duration of at least one contact patch event of a rolling tire.
[0009] Examples provide a method for determining a duration of a
contact patch event of a rolling tire, the method comprising
obtaining a sequence of acceleration measurement samples of the
rolling tire from an acceleration sensor mounted in the tire, and
determining the duration of the contact patch event based on
acceleration measurement samples of the sequence between a first
time instance when the acceleration measurement samples cross a
first threshold and a second time instance when the acceleration
measurement samples cross a second threshold. In some examples, the
duration can be determined based on the samples themselves (for
example, by means of a known sampling rate). The time instances
themselves need not be known.
[0010] In some examples, the slope of the sequence of acceleration
measurement samples crossing the first threshold can be of
different sign than the slope of the acceleration measurement
samples crossing the second threshold. For example, the slope of
the measurement samples crossing the first threshold can be
positive, while the slope of the acceleration measurement samples
crossing the second threshold can be negative.
[0011] In some examples, at least one of the first or second
thresholds can correspond to an average value of the acceleration
measurement samples obtained during one or more rotations of the
tire. The skilled person having benefit from the present disclosure
will appreciate however that different first or second thresholds
can be employed as well.
[0012] In some examples, the first and the second threshold can be
different, similar to a Schmitt trigger. In other example
embodiments, the first and the second threshold may be the
same.
[0013] In some examples, the first time instance can be smaller
than the second time instance and the first threshold can have a
smaller absolute value than the second threshold.
[0014] In some examples, the determination of the duration of the
contact patch event can comprise determining the first time
instance when the acceleration measurement samples cross the first
threshold. The second time instance can be determined when the
acceleration measurement samples cross the second threshold. The
duration can be determined from a difference between the first and
second time instances. Thus, the determination can be made using
only two effective samples. This is computationally cheap and
robust against unexpected changes in the signal waveform.
[0015] In some examples, the determination of the duration of the
contact patch event can be based on the number of samples between
the first and the second time instance and a known sampling rate.
The time instances themselves do not need to be known.
[0016] In some examples, the determination of the duration of the
contact patch event can comprise determining a weighted integral of
the acceleration measurement samples between the first and the
second time instance. The weighting factor can correspond to the
inverse of the difference between the first or the second threshold
and an acceleration value corresponding to zero acceleration.
[0017] In some examples, the first threshold can equal the second
threshold. The first and the second time instance can be determined
by extremizing an integral of the sequence of acceleration
measurement samples with respect to a threshold. The duration of
the contact patch event can be determined by dividing the value of
the integral by a difference between the first or the second
threshold and an acceleration value corresponding to zero
acceleration.
[0018] In some examples, the first threshold can equal the second
threshold. The determination of the duration can comprise
determining a difference between the first or the second threshold
and each sample of the sequence of acceleration measurement
samples. The difference can be accumulated into an accumulated sum.
The accumulated sum can be set to zero whenever the accumulated sum
becomes negative. Accumulating the accumulated sum can be stopped
when the sequence of acceleration measurement samples reaches the
second time instance. The accumulated sum can be divided by the
difference between the second threshold and an acceleration value
corresponding to zero acceleration. Embodiments based on this
method may result in a noise robust, precise, and reproducible
determination of the contact patch duration.
[0019] In some examples, the first time instance may be updated to
the time corresponding to the sample that caused the accumulated
sum to be set to zero. The duration may be determined from the
difference between the first and the second time instances once the
accumulation has stopped.
[0020] In some examples, a first set of samples can be obtained,
processed, and discarded before a subsequent set is obtained. The
skilled person having benefit from the present disclosure will
appreciate that each set may comprise precisely one sample as well
as a plurality of samples (such as those representing a single
rotation of the tire), or some other grouping. For example, the
samples could be obtained, processed, and discarded one by one
before a subsequent sample is obtained. With such embodiments no or
only little memory may be required.
[0021] In some examples, a set equating to one sample can be
obtained, the difference between the sample and the first or the
second threshold determined, the accumulated sum updated by the
difference, and the sample discarded before the next sample (set)
is obtained.
[0022] In some examples, a time window of a subsequent contact
patch event containing the first and the second time instance (or
threshold crossing) of a subsequent contact patch event can be
estimated. The sample rate can be increased during the estimated
time window with respect to a reduced sample rate outside the
estimated time window. This can save energy when outside a contact
patch event.
[0023] In some examples, the time window can be estimated by
determining the rotational rate of the tire, identifying at least
one sample within the sequence of acceleration measurement samples
taken of the rolling tire, indicative of a minimum radial
acceleration and estimating the time window of the subsequent
contact patch event based on the identified sample and the
rotational rate of the tire. The time window can be estimated using
minimal energy and with as little as one sample in the contact
patch.
[0024] In some examples, the estimated time window can be validated
and the method/process can be aborted if the time window exceeds a
predetermined threshold.
[0025] In some examples, the samples and the sequence of samples
can be validated and the method aborted if the samples exceed a
predefined threshold. It should be understood that there are
numerous ways to check the samples, such as by comparing each
sample, the average of the sequence of samples, or the variance of
the sequence of samples, amongst others, to a predetermined
threshold. The method can be aborted and energy saved in the case
of nonsensical data or insufficient signal to noise ratio.
[0026] In some examples, the determination of the duration of the
contact patch event can be validated and the method/process aborted
if the duration exceeds a predetermined threshold.
[0027] In some examples, the determination of the duration of the
contact patch event may be validated by comparing at least two
estimates of the duration of at least one contact patch event
wherein each estimate is obtained by a different estimation
method/process. The method of determining the duration can be
aborted if the at least two estimates differ by more than a
predetermined threshold. For reasonable signal quality, different
methods should yield similar results within a certain accuracy.
Larger differences indicate problematic signal quality, e.g. due to
a pothole, implying that the results should be ignored.
[0028] In some examples, the at least two estimates of the contact
patch duration comprise a first estimate obtained by determining
the first time instance when the acceleration measurement samples
cross the first threshold, determining the second time instance
when the acceleration measurement samples cross the second
threshold after the first time instance, and estimating the
duration from a difference between the first and the second time
instance. A second estimate can be obtained by determining a
difference between the first or the second threshold and each
sample of the sequence of acceleration measurement samples,
accumulating the difference into an accumulated sum, setting the
accumulated sum to zero whenever the accumulated sum is negative,
stopping accumulating the accumulated sum when the sequence of
acceleration measurement samples reaches the second time instance
and dividing the accumulated sum by the difference between the
first threshold and an acceleration value corresponding to zero
acceleration. Additionally, or alternatively, other methods may be
used to obtain the estimates of the duration, such as by
extremizing an integral or a least squares fit.
[0029] According to a further aspect, the present disclosure
proposes a tire-mounted TPMS component. The TPMS component
comprises a tire-mounted acceleration sensor. The acceleration
sensor is configured to generate a sequence of acceleration
measurement samples taken of the rolling tire. The TPMS component
also comprises an electronic control unit configured to determine
the duration of the contact patch event based on acceleration
measurement samples of the sequence between a first time instance
when the acceleration measurement samples cross a first threshold
and a second time instance when the acceleration measurement
samples cross a second threshold.
[0030] In some examples, the electronic control unit can be further
configured to estimate a time window of the subsequent contact
patch event based on the sequence of acceleration measurement
samples. The estimated time window can comprise at least two time
instances corresponding to the first and second time instance of a
subsequent contact patch event. The sensor can further be
configured to increase a sample rate of the sequence of
acceleration measurement samples during the estimated time window
with respect to a reduced sample rate outside the estimated time
window.
[0031] According to a further aspect, the present disclosure
proposes a machine readable storage including machine readable
instructions to determine a duration of a contact patch event of a
rolling tire, that when executed obtains a sequence of acceleration
measurement samples of the rolling tire and determines the duration
of the contact patch event based on acceleration measurement
samples of the sequence between a first time instance when the
acceleration measurement samples cross a first threshold and a
second time instance when the acceleration measurement samples
cross a second threshold.
[0032] As used herein, a tire may be, in addition to any common
usage in the art, any deformable rotating device, particularly one
that deforms when it comes in contact with a surface. A tire does
not have to be made of rubber or any particular material.
BRIEF DESCRIPTION OF THE FIGURES
[0033] Some examples of apparatuses and/or methods will be
described in the following by way of example only, and with
reference to the accompanying figures, in which
[0034] FIG. 1 shows a schematic cross-section of a tire with a
tire-mounted TPMS module;
[0035] FIG. 2 shows a representative graph of the radial
acceleration profile of the tire;
[0036] FIG. 3 shows a flowchart of a method for determining the
duration of a contact patch event of a tire;
[0037] FIG. 4A shows an example sequence of acceleration
measurement samples;
[0038] FIG. 4B shows a flowchart of a trigger method for
determining the duration of a contact patch event;
[0039] FIG. 5A shows another example sequence of acceleration
measurement samples;
[0040] FIG. 5B shows a flowchart of an area estimation method for
determining the duration of a contact patch event;
[0041] FIG. 6 shows a graph comparing the accuracy of three
different methods for determining the duration of a contact patch
event;
[0042] FIG. 7 shows a flow chart of a method to vary the sampling
rate based on estimated need;
[0043] FIG. 8 shows a flow chart of a method to validate the
determination of the duration of a contact patch event;
[0044] FIG. 9 shows an exemplary flowchart of a method for
predicting and determining the duration of the contact patch
event.
DETAILED DESCRIPTION
[0045] Various examples will now be described more fully with
reference to the accompanying drawings in which some examples are
illustrated. In the figures, the thicknesses of lines, layers
and/or regions may be exaggerated for clarity.
[0046] Accordingly, while further examples are capable of various
modifications and alternative forms, some particular examples
thereof are shown in the figures and will subsequently be described
in detail. However, this detailed description does not limit
further examples to the particular forms described. Further
examples may cover all modifications, equivalents, and alternatives
falling within the scope of the disclosure. Like numbers refer to
like or similar elements throughout the description of the figures,
which may be implemented identically or in modified form when
compared to one another while providing for the same or a similar
functionality.
[0047] It will be understood that when an element is referred to as
being "connected" or "coupled" to another element, the elements may
be directly connected or coupled or via one or more intervening
elements. If two elements A and B are combined using an "or", this
is to be understood to disclose all possible combinations, i.e.
only A, only B as well as A and B. An alternative wording for the
same combinations is "at least one of A and B". The same applies
for combinations of more than 2 Elements.
[0048] The terminology used herein for the purpose of describing
particular examples is not intended to be limiting for further
examples. Whenever a singular form such as "a," "an" and "the" is
used and using only a single element is neither explicitly or
implicitly defined as being mandatory, further examples may also
use plural elements to implement the same functionality. Likewise,
when a functionality is subsequently described as being implemented
using multiple elements, further examples may implement the same
functionality using a single element or processing entity. It will
be further understood that the terms "comprises," "comprising,"
"includes" and/or "including," when used, specify the presence of
the stated features, integers, steps, operations, processes, acts,
elements and/or components, but do not preclude the presence or
addition of one or more other features, integers, steps,
operations, processes, acts, elements, components and/or any group
thereof.
[0049] Unless otherwise defined, all terms (including technical and
scientific terms) are used herein in their ordinary meaning of the
art to which the examples belong. In the following figures optional
components, actions or steps are shown in broken lines.
[0050] FIG. 1 shows a schematic cross-section of a tire 104 with a
tire-mounted TPMS module 100. The tire 104 with the tire-mounted
TPMS module 100 rolls on a surface or road. The tire 104 forms a
contact patch 102 with the road 101 as it rotates. An angle 103 is
formed between the tire-mounted module 100 and the normal to the
road. The angular position .PHI. can be defined as the angle
spanning between the TPMS module and the vertical axis. A "contact
patch event" occurs when the tire-mounted module 100 is located in
the contact patch 102. In other words, the contact patch event
occurs when the outer surface of the tire where the module 100 is
mounted touches the road surface 101. The contact patch length is
the length of the tire that flattens within the contact patch 102.
The contact patch duration is the amount of time that the TPMS
module 100 is within the contact patch 102 during rotation of the
tire.
[0051] FIG. 2 is a representative graph 200 of the radial
acceleration profile of the tire plotted against the angular
position (in degrees). FIG. 2 shows a typical acceleration-signal
acquired by a radial accelerometer or acceleration sensor of a
tire-mounted TPMS module. Ignoring higher frequency components,
most of the data is constant and lies close to a baseline or
average 201 (close to 400 m/s.sup.2 for the exemplary signal in
FIG. 2), when the tire-mounted TPMS module 100 is not in the
contact patch. As the module 100 enters the contact patch event, a
sharp spike in the acceleration profile occurs at 202 followed by a
near-zero reading during the contact patch event 203.
[0052] The acceleration of a tire-mounted TPMS module is nearly
constant for the largest part of the tire revolution (apart from
mechanical vibrations). In this part, the acceleration is mainly
determined by the centrifugal acceleration. The centrifugal
acceleration a.sub.cf on a circular trajectory with a radius R and
a velocity v is given by the equation
a.sub.cf=v.sup.2/R. (1)
[0053] In the contact patch, however, when the module 100 is close
to the road surface, the acceleration experienced by the TPMS
module 100 is nearly zero. Shortly before entering and leaving the
contact patch 102, the tire has to deform significantly. This
increases the local curvature of the TPMS module's trajectory.
Thus, the experienced acceleration is also increased.
[0054] Under slip-free conditions, the tire itself rolls over
sections which touch the road surface (i.e. the contact patch),
while these sections are virtually stationary. Thus, a TPMS module
experiences virtually no acceleration when passing through this
contact patch (i.e. the contact patch event). Further assuming a
freely rolling wheel, i.e. a wheel on which no torque is applied,
this contact patch coincides with the angular position defined as
.PHI.=0 (i.e. the angle formed normal to the ground). Because the
nearly vanishing acceleration during the contact patch event is so
prominent, the subsequent angular position and the duration of the
contact patch event can be estimated.
[0055] FIG. 3 shows a flow chart of a method 300 for determining
the duration of a contact patch event of a rolling tire. The method
300 comprises obtaining 310 a sequence of acceleration measurement
samples of the rolling tire from a tire-mounted acceleration
sensor. Method 300 further comprises determining or estimating 320
the duration of the contact patch event based on acceleration
measurement samples of the sequence between a first time instance
when the acceleration measurement samples cross a first threshold
and a second time instance when the acceleration measurement
samples cross a second threshold.
[0056] The acceleration measurement samples may be directly
measured or generated by the tire-mounted acceleration sensor
during rotation of the tire and output from the sensor interface to
a control unit. By directly measuring and processing the samples,
minimal memory may be used. In other examples, the samples may be
stored in memory and recalled later for processing. The control
unit may be tire-mounted, but in other examples it may be located
at a remote location. It should be appreciated that the
acceleration measurement samples do not necessarily need to measure
the acceleration itself, but could instead be any measured quantity
indicative of the (radial) acceleration (i.e. any quantity from
which the acceleration component could be derived or determined
could correspond to an acceleration measurement sample).
[0057] It should be appreciated that although samples taken during
at least one complete rotation of the rolling tire guarantee that
contact patch data is sampled, it is possible to determine the
duration without sampling a complete a rotation of the rolling
tire. In some examples, it is possible to predict the contact patch
event and to take samples only during the predicted contact patch
event, saving energy. In some examples, it is possible to take and
process samples on the fly until the duration or the second time
instance are determined, at which point the sampling can be
stopped, again saving energy.
[0058] One of skill in the art can understand that the duration may
be determined or estimated by a variety of methods based on the
acceleration measurement samples. As will become apparent in the
remainder of this disclosure, there are various concepts for
explicitly or implicitly determining the first and the second time
instances. For example, if the number of the samples between the
first and second time instance are known, then one only needs the
sampling rate (which is known) in order to estimate the duration.
The time instances themselves do not need to be known explicitly.
On the other hand, if one knows the actual time of the first and
second instances, then the duration can be found by trivial
subtraction. Some methods may rely on a combination of the above
information as explained in detail below.
[0059] Due to the limited power and memory of the tire-mounted TPMS
component, techniques which are computationally efficient, light on
memory, and reduce power usage are highly desired. Examples
comprise, amongst others, a trigger method, an area estimation
method, an integration method, and a least square fit method each
providing a different computational efficiency and accuracy. One
skilled in the art can appreciate that numerous variations to these
methods may be implemented.
[0060] FIG. 4A shows an example sequence of acceleration
measurement samples corresponding to a fraction of one tire
revolution, which could also be referred to a frame of acceleration
measurement samples. As can be appreciated from this example, a
complete tire revolution can comprise tens, hundreds or even
thousands of discrete acceleration measurement samples. As can be
seen from FIG. 4A, a first threshold 410 (Lower Trigger Threshold,
LTT) and a second threshold 412 (Upper Trigger Threshold, UTT) can
be defined. The UTT 412 can be set to an upper boundary. In the
example of FIG. 4A, it is set equal to an afore determined average
acceleration. The LTT 410 can be set to a value between the UTT and
a value corresponding to zero acceleration. In the example of FIG.
4A, the LTT 410 is set equal to 1/4 of the UTT 412 (i.e. 1/4 of the
average). The skilled person having benefit from the present
disclosure will appreciate, however, that other threshold values
are possible and even beneficial in other implementations.
[0061] FIG. 4B illustrates an example of the trigger method 400,
also called the Schmitt trigger method. Examples of the trigger
method 400 comprise determining 402 a first time instance 414 when
the acceleration measurement samples cross the first threshold
(LTT) 410. After the first time instance 414 is determined, a
second time instance 416 when the acceleration measurement samples
cross the second threshold (UTT) 412 is determined 404 as well.
Then, the duration can be determined in 406 from a difference
between the first and the second time instance.
[0062] The skilled person having benefit from the present
disclosure will appreciate that determining the first and second
time instances can be done explicitly or implicitly. For example,
the first/second time instance can be determined by considering a
sample number of the acceleration measurement sample crossing the
first/second threshold together with the sample rate. In other
implementations, only the number of samples in between the two
threshold crossings may be considered. The duration can then be
determined by multiplying the number of samples in between the two
threshold crossings with the sample rate.
[0063] An example of method 400 may be summarized by the following
pseudo-code, where UTT corresponds to the upper trigger (second)
threshold 412, LTT corresponds to the lower trigger (first)
threshold 410, and acc corresponds to the acceleration measurement
sample being analyzed:
TABLE-US-00001 Sample until acc > UTT Sample until acc < LTT
index1 Sample until acc > UTT index2 length .varies. index2 -
index1 If length < min_length then return to step 2.
[0064] The trigger method relies on the two thresholds, 410 and
412. Acceleration measurement samples smaller than the first (LTT)
threshold 410 turn the trigger on. Acceleration measurement samples
larger than the second (UTT) threshold 412 turn the trigger off.
The trigger maintains its state in between. Thus, while the trigger
is off the method checks for acc<LTT and while the trigger is on
the method checks for acc>UTT. Finally, the duration is compared
to a predetermined reasonable value to ensure validity of the data.
Adjustable minimal and maximal contact patch durations can be used
to increase the robustness against noise and other disturbances in
the acceleration signal.
[0065] In performing the trigger method 400, the trigger initially
begins in the off state. In the off state, the acceleration samples
are checked until a sample crosses the first (LTT) threshold 410,
as seen by point 414 in FIG. 4A. Once a sample crosses below the
LTT, the first time instance 414 is noted and the trigger is
changed to the on state. In the on state, samples are checked until
a sample crosses above the second (UTT) threshold 412, as seen by
point 416 in FIG. 4A. Once a sample crosses above the UTT while the
trigger is turned on, the second time instance 416 is noted and the
trigger is turned off. Thus, the duration can be determined by a
time difference between the first and second time instances, as
shown below the graph by 418.
[0066] As can be seen in FIG. 4A, the slope of the sequence
crossing the first threshold 410 is of a different sign than the
slope of the sequence crossing the second threshold 412. As can
also be seen, the first threshold 410 has a smaller absolute value
than the second threshold 412, and the first time instance 414
occurs before the second time instance 416.
[0067] The upside of the trigger method 400 is that it is robust
against unexpected changes in the signal waveform and
computationally cheap. On the other hand, its result is only based
on two effective samples, thus considerably influenced by
noise.
[0068] To minimize memory and power use, the samples can be
analyzed after each sample is measured and the sample discarded. By
analyzing and discarding each measurement sample as it is taken,
one can avoid storing the entire sequence in memory. Alternatively,
one can measure a set of samples at time, wherein a set can
correspond to a single sample only or to samples taken within a
certain period (such as one rotation of the tire).
[0069] FIG. 5A shows another example sequence of acceleration
measurement samples corresponding to a fraction of one tire
revolution. Here, the first and the second threshold are both set
equal to a single threshold 520 corresponding to the average
acceleration (similar to the UTT of the trigger method). Another
option to determine the duration of the contact patch event is to
determine the duration between a first time instance 524 when the
acceleration measurement samples fall below the threshold 520 and a
second time instance 526 when the acceleration measurement samples
exceed the threshold 520. This can involve integrating or
accumulating the acceleration measurement samples.
[0070] FIG. 5B shows an example of an area estimation method 500,
wherein the first and second thresholds are the same (threshold
520). This method involves first an act 502 of finding the
difference between the threshold 520 and each measured acceleration
sample. In a next act 504 these differences are accumulated into an
accumulated sum. If the accumulated sum becomes negative, then it
is reset to zero (see 506) and the accumulation restarts from 0
with the subsequent acceleration measurement sample. The first time
instance 524 can implicitly correspond to the last accumulation
restart. The accumulation 508 continues until the second time
instance 526 is reached.
[0071] One skilled in the art can appreciate that the second time
instance 526 can be determined by a number of different methods;
one such method is by using the second time instance as found in
the trigger method as described above.
[0072] Once the second time instance 526 is reached and the
accumulation has been stopped, the accumulated sum represents the
area under the graph with respect to the threshold 520. The height
of this area corresponds to a difference between threshold 520 and
value 522 corresponding to zero acceleration. The width of this
area corresponds to time. Since area equals height times width, we
can divide the accumulated area by the height to obtain the width.
Thus, one can divide the accumulated sum by the difference between
the threshold 520 and the acceleration value 522 corresponding to
zero acceleration (i.e. the height) to obtain the time duration of
the contact patch event.
[0073] The area estimation method 500 may be summarized by the
following pseudo-code, wherein acc_sum corresponds to the
accumulated sum, avg corresponds to the average of the acceleration
measurement samples (i.e. the first or second threshold) and acc
corresponds to the acceleration measurement sample being
analyzed:
TABLE-US-00002 Set acc_sum = 0 For every sample: acc_sum += (avg -
acc) If acc_sum < 0 then acc_sum = 0 After summation: duration =
acc_sum/avg
[0074] To minimize memory and power use, the samples can be
analyzed after each sample is measured, the difference between the
threshold and the measured sample determined, this value added to
the accumulated sum and the sample discarded. By analyzing and
discarding each measurement sample as it is taken, one can avoid
storing the entire sequence in memory. Alternatively, one can
measure a set of samples at time, wherein a set can correspond to a
single sample only or to samples taken within a certain period
(such as one rotation of the tire).
[0075] In performing the method 500, the difference between the
threshold 520 and the measured samples is found and this value is
accumulated. As can be seen by the graph of the samples in FIG. 5A,
this accumulated sum will be negative for samples before the sample
labeled at 524. Thus, the accumulated sum will be repeatedly set to
zero until the sample at point 524.
[0076] After the sample at 524, the accumulated sum begins to
become positive. Although a small negative peak 528 exists shortly
after 524 (at approximately -3 ms), the area of this peak is not
large enough to negate the accumulated sum, and thus, the
accumulated sum is not reset to zero. The accumulation continues
until the second time instance 526 (corresponding to time 416 of
the trigger method).
[0077] Finally, the accumulated area is divided by the height (i.e.
the difference between 520 and 522) leading to the duration
528.
[0078] Additionally, or alternatively, the first time instance may
be updated to correspond to the most recent sample which causes the
accumulated sum to be set to zero. Thus, the duration may be
determined by the difference between the first and second time
instances without needing the value of the accumulated sum
explicitly.
[0079] It should be appreciated that other methods for determining
the duration may be possible, but the above-mentioned methods are
particularly efficient and take into consideration the limited
resources available at the tire-mounted TPMS component. Other
methods may comprise, for example, extremizing the integral or
fitting the data according to the least square fit method. These
methods may provide more accurate results, but they are not always
practical to use given the limited resources of tire-mounted TPMS
components.
[0080] FIG. 6 shows a representative graph 600 of the results of
three different methods of obtaining the durations of contact patch
events. The first method 601, labeled by triangles, shows the use
of a traditional Least Square Fit method which is computationally
intensive, but provides a high-level of accuracy. The second method
602, labeled by X's, shows the use of the trigger method as
outlined in 400, above. Being computationally cheap and
conceptually simple, this method is influenced by noise to a
considerable degree and thus not as precise, but efficient to run.
The third method 603, labeled by circles, shows the use of the area
estimation method as outlined in 500, above. This method reaches a
precision comparable with the computationally demanding Least
Square Fit method 601. The systematic differences between the three
methods stem from the different threshold levels, at which the
"contact patch durations" are evaluated by the different methods.
The examples denoted in graph 600 are representative of the
exemplary methods denoted above; one skilled in the art can
appreciate that deviations from these methods can occur which can
result in over- or under-estimations of their results with varying
computational needs.
[0081] The above methods require a high sampling frequency but
relatively few computations per sample. While this enables
processing the data during sample acquisition, switching the
processing circuit to a low-power state while performing these
methods might be inefficient. In order to minimize power use, it
would be better to run these methods for as short as possible;
ideally starting immediately before a contact patch. Predicting the
next contact patch event allows one to wait in a low-power mode for
the expected event, and only then execute the method in high-power
mode.
[0082] FIG. 7 shows a flow chart of an optional method for varying
the sample rate while performing the method 300. First, a time
window 710 is estimated corresponding to a subsequent contact patch
event.
[0083] One way to estimate this time window is by 712 determining a
rotational rate of the tire. By knowing the rotational rate of the
tire, one can predict when the tire will be in the same position on
its next rotation.
[0084] One way to determine the information on the rotational rate
of the tire may be by deriving T.sub.rev from the average radial
acceleration <a> and the geometrical tire radius R via the
formula
T.sub.rev=2.pi. {square root over (R/a)}. (2)
[0085] For tire-mounted TPMS modules, the average radial
acceleration agrees reasonably well with the centrifugal
acceleration calculated from Eq. Error! Reference source not
found., where R is approximated by the geometrical tire radius and
v by the tire's velocity. Therefore, the velocity can be calculated
from the average radial acceleration. Without slip, the velocity v
is related to the period of the revolution T.sub.rev and the
effective tire radius R.sub.eff via the equation
v=2.pi.R.sub.eff/T.sub.rev. (3)
[0086] For a well-inflated tire, this effective radius is only
marginally smaller than the geometrical radius. Thus, set
R.sub.eff=R in Eq. (3), and calculate T.sub.rev from the average
acceleration <a> according to Eq. (2). Instead of the
arithmetic mean <a>, in other examples one could use the
median in this equation. This could improve the robustness against
outliers at slightly increased computational demand.
[0087] Next, 714 identify within a sequence of acceleration samples
at least one sample indicative of a minimum radial acceleration
(i.e. at least one sample corresponding to the zero acceleration
dip as shown in FIG. 2 at 203). Thus, 716 a time window can be
estimated based on the position of the contact patch event and the
known rotational rate of the tire.
[0088] It should be appreciated that there are numerous ways to
estimate this time window aside from using the rotational rate. For
example, if one knows the contact patch events of at least two
consecutive rotations of the tire, then the time difference between
these two events may be used as an estimate for the subsequent
contact patch.
[0089] Once a time window 710 has been estimated, an optional 720
validation check may be performed. The estimated time window may be
checked against a predetermined duration and the method aborted if
the estimated window is outside the predetermined duration. For
example, if the estimated time window is longer than the time it
takes for the tire to perform a complete rotation, it can be
immediately determined that the estimate is wrong and the method
aborted to prevent erroneous data and to save energy.
[0090] Once a time window 710 has been estimated and (optionally)
validated, the 730 sampling rate may be increased during the
estimated time window with respect to a reduced sampling rate
outside the time window. Thus, the method may obtain samples at a
sufficiently high sampling rate during the predicted contact patch
event while saving energy and avoiding unnecessary samples outside
of the predicted contact patch event.
[0091] FIG. 8 shows a flow chart of an optional method for 810
validating the determination of the duration as obtained from
method 300 at 320. The method may be aborted if the determined
duration exceeds a predetermined threshold. For example, general
data representative of contact patch lengths (or durations)
corresponding to various loads on a tire may be determined using a
test rig, and compared to the durations as determined by the method
300.
[0092] One method for validating the determination of the duration
may comprise 812 comparing two different determinations obtained by
two different methods for the same contact patch event of the tire.
Since the two determinations represent the same contact patch
event, they should be within an error threshold of each other. If
the two determinations differ by more than a predetermined amount,
then at least one determination is erroneous and the method may be
aborted. An example of the comparison 812 may comprise a first
determination using the trigger method 400 and a second
determination using the area estimation method 500. It should be
noted that, while the trigger and area estimation methods are
provided as examples, any two determinations obtained using
different methods may be used to 810 validate the
determination.
[0093] FIG. 9 shows a flowchart of an exemplary method employing
numerous optional steps as described above. The method begins by
first 710 estimating a time window of a subsequent contact patch
event. The time window is estimated by 712 determining a rotational
rate of the tire and 714 identifying a sample indicative of a
minimal acceleration. The 716 time window is estimated based on the
712 rotational rate and 714 identified sample.
[0094] Next, the data is validated. This validation may encompass
one or many validation steps. For example, the acquired samples may
be checked against predetermined thresholds individually or as a
sequence. Additionally, or alternatively, the 720 estimated time
window may be validated. As with all validation steps, if the data
exceeds a predetermined error threshold, the method is aborted to
save power and avoid nonsensical or extraneous data (such as those
due to uneven road conditions or other unexpected forces).
[0095] Once an appropriate time window estimate is achieved, the
730 sampling rate is modified accordingly such that a high sampling
rate is achieved during the subsequent contact patch event while a
reduced sampling rate is used otherwise. Thus, the system stays in
low power (i.e. low sampling rate) until the estimated time
arrives. This is shown by PDWN (IT) which represents the powering
down of unnecessary circuits, with a wake-up scheduled by an
internal timer according to the estimated time window.
[0096] Once the estimated contact patch approaches, the system
switches to high sampling rate and begins to perform the method
300. As explained above, this begins by 310 obtaining samples and
then 320 determining the duration based on the samples using one or
more of the aforementioned methods (such as the 400 trigger and/or
500 area estimation methods).
[0097] Finally, once a duration is determined, it is also
validated. As explained above, the 810 validation of the duration
may comprise 812 a comparison of determinations using at least two
different methods for the same contact patch event. As with all
validation steps, if the two determinations vary by more than a
predetermined threshold, the method is aborted.
[0098] In performing any of the above methods, one skilled in the
art can appreciate that the thresholds can be changed, which might
result in an over- or under-estimation of the contact patch
duration. For example, one can move the first and second threshold
of the trigger method further way from each other, which will
result in a larger duration determination (and thus an
over-estimate). One can move the thresholds closer together, which
will result in a smaller duration determination (and thus an
under-estimate). Similar changes can be made to any of the above
described methods.
[0099] The aspects and features mentioned and described together
with one or more of the previously detailed examples and figures,
may as well be combined with one or more of the other examples in
order to replace a like feature of the other example or in order to
additionally introduce the feature to the other example.
[0100] Examples may further be or relate to a computer program
having a program code for performing one or more of the above
methods, when the computer program is executed on a computer or
processor. Steps, operations or processes of various
above-described methods may be performed by programmed computers or
processors. Examples may also cover program storage devices such as
digital data storage media, which are machine, processor or
computer readable and encode machine-executable,
processor-executable or computer-executable programs of
instructions. The instructions perform or cause performing some or
all of the acts of the above-described methods. The program storage
devices may comprise or be, for instance, digital memories,
magnetic storage media such as magnetic disks and magnetic tapes,
hard drives, or optically readable digital data storage media.
Further examples may also cover computers, processors or control
units programmed to perform the acts of the above-described methods
or (field) programmable logic arrays ((F)PLAs) or (field)
programmable gate arrays ((F)PGAs), programmed to perform the acts
of the above-described methods.
[0101] The description and drawings merely illustrate the
principles of the disclosure. Furthermore, all examples recited
herein are principally intended expressly to be only for
pedagogical purposes to aid the reader in understanding the
principles of the disclosure and the concepts contributed by the
inventor(s) to furthering the art. All statements herein reciting
principles, aspects, and examples of the disclosure, as well as
specific examples thereof, are intended to encompass equivalents
thereof.
[0102] A functional block denoted as "means for . . . " performing
a certain function may refer to a circuit that is configured to
perform a certain function. Hence, a "means for s.th." may be
implemented as a "means configured to or suited for s.th.", such as
a device or a circuit configured to or suited for the respective
task.
[0103] Functions of various elements shown in the figures,
including any functional blocks labeled as "means", "means for
providing a sensor signal", "means for generating a transmit
signal.", etc., may be implemented in the form of dedicated
hardware, such as "a signal provider", "a signal processing unit",
"a processor", "a controller", etc. as well as hardware capable of
executing software in association with appropriate software. When
provided by a processor, the functions may be provided by a single
dedicated processor, by a single shared processor, or by a
plurality of individual processors, some of which or all of which
may be shared. However, the term "processor" or "controller" is by
far not limited to hardware exclusively capable of executing
software, but may include digital signal processor (DSP) hardware,
network processor, application specific integrated circuit (ASIC),
field programmable gate array (FPGA), read only memory (ROM) for
storing software, random access memory (RAM), and non-volatile
storage. Other hardware, conventional and/or custom, may also be
included.
[0104] A block diagram may, for instance, illustrate a high-level
circuit diagram implementing the principles of the disclosure.
Similarly, a flow chart, a flow diagram, a state transition
diagram, a pseudo code, and the like may represent various
processes, operations or steps, which may, for instance, be
substantially represented in computer readable medium and so
executed by a computer or processor, whether or not such computer
or processor is explicitly shown. Methods disclosed in the
specification or in the claims may be implemented by a device
having means for performing each of the respective acts of these
methods.
[0105] It is to be understood that the disclosure of multiple acts,
processes, operations, steps or functions disclosed in the
specification or claims may not be construed as to be within the
specific order, unless explicitly or implicitly stated otherwise,
for instance for technical reasons. Therefore, the disclosure of
multiple acts or functions will not limit these to a particular
order unless such acts or functions are not interchangeable for
technical reasons. Furthermore, in some examples a single act,
function, process, operation or step may include or may be broken
into multiple sub-acts, -functions, -processes, -operations or
-steps, respectively. Such sub acts may be included and part of the
disclosure of this single act unless explicitly excluded.
[0106] Furthermore, the following claims are hereby incorporated
into the detailed description, where each claim may stand on its
own as a separate example. While each claim may stand on its own as
a separate example, it is to be noted that--although a dependent
claim may refer in the claims to a specific combination with one or
more other claims--other examples may also include a combination of
the dependent claim with the subject matter of each other dependent
or independent claim. Such combinations are explicitly proposed
herein unless it is stated that a specific combination is not
intended. Furthermore, it is intended to include also features of a
claim to any other independent claim even if this claim is not
directly made dependent to the independent claim.
* * * * *